1. Field of the Invention
This invention relates to a method for applying a concentric coating to a filament and more particularly, to a method for coating a glass optical waveguide filament.
Glass optical waveguide filaments must exhibit high strength in order to withstand the stresses which are encountered in incorporating them into protective sheathing or cable, installing the cable, or in use. While such waveguides are typically quite strong as drawn from a preform or blank, this strength is rapidly degraded by surface defects which are introduced into the waveguide through handling or otherwise.
To preserve the strength of a newly drawn waveguide filament, it is conventional to apply to the waveguide immediately after it is drawn a thin protective coating composed of an organic or inorganic coating material which serves to shield the waveguide during subsequent handling.
2. Description of the Prior Art
Tapered dies have been employed for the application of coatings to filaments of non-brittle materials such as conductive wires. U.S. Pat. No. 4,093,414 issued to E. S. Swiatovy, Jr. teaches an apparatus employing a single extruder die for applying two coatings to a conductive wire which is maintained in a central position within the extrusion die by a tip having a wire passage extending lengthwise therethrough. Since the conductive wire is not adversely affected by contacting surfaces within the coating apparatus prior to the application of the coating, the tip orifice is just large enough to permit the conductive wire to pass therethrough.
In the field of glass optical waveguides it is desirable to coat the glass filament with a plastic in order to preserve filament strength. In accordance with the teachings of U.S. Pat. No. 3,960,530 issued to R. Iyengar a glass filament is coated on the draw to prevent moisture and dust from contaminating the surface thereof between the time that it is drawn and the time that it is coated and to ensure that the filament surface is coated before microcracks which are on the surface thereof begin to enlarge. The coating apparatus of the Iyengar patent comprises a lower die closure having an aperture therein and a nozzle centrally disposed within the aperture for guiding the glass filament. The lower portion of the die aperture and the outer surface of the nozzle form a cylindrical passage through which the coating material flows prior to its application to the filament. Since the filament is approximately the same size as the nozzle aperture, the filament is subjected to abrasion from the nozzle aperture surface.
U.S. Pat. No. 4,116,654 issued to Y. M. Trehu relates to a method and apparatus for coating a silica filament while ensuring that the filament does not contact any solid surface within the coating die prior to coating. The silica filament passes through the tip portion of a filament guide which is recessed between 0.5 and 1 mm within the extrusion orifice. The amount of recess is sufficient to allow for some flow of molten polymer up into the narrow bore of the tip portion. The clearance between the filament and the inside of the narrow bore is made large enough to allow polymer to flow into the bore but is said to be small enough to maintain good concentricity between core and cladding. For reasons which will be explained hereinafter, the upward flow of coating material into the bore of a filament guide may adversely affect the centering of the filament within the coating.
Copending U.S. patent application Ser. No. 46,232 of J. W. Ohls for "Method of Coating Optical Waveguide Filaments and Coating Die" filed June 7, 1979, teaches a method of coating an optical waveguide filament employing a die body having an at least partly tapered central aperture and radial means for introducing coating material to the central aperture. The method includes exposing the optical waveguide filament to the coating material within the tapered aperture of the die body. The method of the Ohls application does not employ any means separate from the sizing die for centering purposes.